Heretofore, Advanced communications today use the Orthogonal Frequency Division Multiplex (OFDM) modulation for efficient transmission of digital signals. These signals may include video, voice and/or data. OFDM is a commonly used implementation of Multi-Carrier Modulation (MCM).
The Orthogonal Frequency Division Multiplex (OFDM) is a modern advanced modulation method, that achieves better use of the frequency spectrum, as detailed below.
OFDM has been used in recent years in many applications where robustness against severe multipath and interference conditions is required, together with high system capacity, flexibility in providing variable bit rate services, scalability and a capability to perform well in Single Frequency Networks (SNF). OFDM forms the basis for various communication standards, including for example the Digital Terrestrial Television Broadcasting, Digital Audio Broadcasting (DAB), wireless LANs and Wireless Local Loops.
A particular example of the use of OFDM is in DVB systems. Digital video broadcasting (DVB) systems are now being developed, based on several standards, in Europe, U.S.A. and Japan. Each standard addresses cable broadcasting as well as satellite and terrestrial/area broadcasting.
In Europe, various standards define the digital video broadcasting (DVB) system, including DVB-T (DVB-Terrestrial), DVB-C (DVB-Cable) and DVB-S (DVB-Satellite). For example, the European standard EN 300 744 defines the DVB-T.
The OFDM modulation method has been chosen, for example, for the digital television broadcasting (MPEG-2) per standard DVB-T.
A disadvantage of presently used DVB-T systems is their unidirectional operation. That is, information is only transmitted from a base station (the transmitter) to subscribers (the receivers). A system may contain many base stations and subscribers, however there is always an unidirectional flow of information.
An interactive system can be achieved by using a telephone line as a return channel from the set top box, however this method is slow and inconvenient.
A fast, flexible link from subscriber to the base station is required for the multitude of advanced services that are in demand today.
To achieve a bi-directional link is a difficult task, that requires an innovative approach as detailed below. The problem is further aggravated by the requirement that the solution should cause no deterioration in performance; moreover, updated subscriber units capable of bi-directional operation should coexist with older subscriber units that do not have these transmission capabilities.
To understand the difficulty of complying with these requirements, one should delve into the intricacies of contemporary digital video broadcasting standards. These standards specify advanced signal processing, to achieve higher quality communications at very high bit rates.
Thus, in the Orthogonal Frequency Division Multiplex (OFDM) modulation method, a block of information is divided among N frequency channels, so that a portion of the information is transmitted in each of the abovementioned channels or frequencies. Since each channel is orthogonal to the others, a better utilization of the frequency spectrum is achieved.
The OFDM method achieves lower Inter-Symbol Interference ISI, since the distribution of the information over N carriers allows each bit of information to be sent for a longer time period (N times longer). For a low ISI, the overlap between adjacent symbols should be lower than 10%. The ISI increases as the percentage of overlap between adjacent symbols increases. In OFDM, since each symbol is N times longer, the percent overlap between adjacent symbols decreases, hence the Inter-Symbol Interference ISI is lower. Still better spectrum utilization is achieved by QAM (Quadrature Amplitude Modulation) on each of the N carriers.
An IFFT (Inverse Fourier Transform) is performed on the modulated carriers, to form the signal in the time domain that corresponds to the above modulated carriers. The signal is transmitted as a frame that contains the block of information to be transmitted.
A possible problem with the above modulation method is multipath, that may result in interference between adjacent transmitted frames. To address this problem, a guard time period is inserted between adjacent frames. The guard time is especially important in QAM, that is more sensitive to interference. In DVB-T systems, the guard time is chosen as either ¼, ⅛, 1/16 or 1/32 of the symbol time.
A disadvantage of presently used OFDM channels is the need to reserve a guard interval in order to battle multipath and to enable operation in SFN networks. The guard interval, which is up to 25% of the symbol duration, is in effect a wasted time, because no information is transmitted during that time interval.
Although the guard time is used to address the multipath problem, it is a costly solution, since it reduces the capacity of the communication system. It would be highly desirable to use other means for solving the multipath problem, that would allow channel operation at full speed.
Therefore, it is a formidably difficult task to try and improve or change these complex communications standards.
Several methods are now used to separate signals transmitted over a common channel, including:                TDMA—Time Division Multiple Access        FDMA—Frequency Division Multiple Access        DS-CDMA—Direct Sequence/Code Division Multiple Access        
CDMA systems may use either DS/CDMA or FH/CDMA. In DS/CDMA multicode or DS/Multicode/CDMA, the separation of signals transmitted over a common channel is achieved using orthogonal codes.
At present, a problem in DS/CDMA is how to generate these orthogonal codes. It is possible to have N channels using orthogonal Walsh codes to multiply each channel, wherein each user has a different Walsh code. In the downlink channel (DL), that is the channel from the base station to subscribers, the orthogonality is preserved, since transmission to all users is prepared and transmitted at the same time. Each user receives all the encoded messages at the same time.
In the uplink, however, each user has a different timing because of a different propagation time delay. Thus, each Walsh code (corresponding to a specific user) may be shifted in time relative to the other codes (that correspond to the other users). This effect creates interference between channels. The problem is further aggravated by multipath, that may cause the phase shift in each channel to change in time.
In prior art CDMA, alignment up to a portion of one chip was required to maintain orthogonality between signals. This is a severe requirement, that affects the cost and complexity of the communication equipment.
A reduced orthogonality may cause a higher level of inter-user interference, caused by cross-correlation effects.
Prior art systems apparently do not disclose a system similar to that detailed in the present disclosure.
Thus, Seki et al., U.S. Pat. No. 5,771,224, discloses an orthogonal frequency division multiplexing transmission system and transmitter and receiver therefor. It transmits an OFDM transmission frame, with null symbols and reference symbols being placed in the beginning portion of the frame and QPSK symbols are placed in an information symbol data region in the frame, with equal spacing in time and frequency.
The carrier amplitude and phase errors are corrected by a correction information producing section on the amplitude and phase variations of the received signal detected by the variation detector to produce corrected information.
Apparently, Seki does not address the problem of reverse link transmissions. Moreover, Seki performs a different type of signal processing.
Baum et al., U.S. Pat. No. 5,802,044, discloses a multicarrier reverse link timing synchronization system. A center station transmits a forward link signal, receives a reverse link signal, and determines a timing offset for signals received on a reverse link timing synchronization channel. A reverse link symbol timing synchronization can be used in a system having a plurality of transmitting overlap bandwidth subscriber units on an OFDM-like spectrally overlapping reverse channel. The modulation method may comprise M-ary Quadrature Phase Shift Keying (M-PSK), M-ary Quadrature Amplitude Modulation (QAM) or other digital modulation method.
Gudmundson et al., U.S. Pat. No. 5,790,516, discloses a method and system for pulse shaping for data transmission in an orthogonal frequency division multiplexed (OFDM) system.